1. Field
The present disclosure relates generally to communications and, in particular, to satellite communications. Still more particularly, the present disclosure relates to a method and apparatus for reducing interference with satellite communications.
2. Background
Many different types of satellites are present for different purposes. For example, satellites include observation satellites, communication satellites, navigation satellites, weather satellites, research satellites, and other suitable types of satellites. Additionally, space stations and human spacecraft in orbit are also satellites that may perform different purposes.
With respect to satellites, communication of information is performed by most satellites. Communications may include receiving information and transmitting information. The information received may be commands, data, programs, and other types of information. Information transmitted by satellites may include data, images, communications, and other types of information.
When a satellite is primarily used to relay communications, the satellite may relay information to different destinations across the Earth using signals. In these illustrative examples, the signals are used to establish a communications link between the satellite and another device. Typically, when communications are sent to a satellite, the communications link is in an uplink. Information transmitted by a satellite is typically in a downlink.
For example, a transmitter in one location may send information in a communications link in the form of an uplink to a satellite. The satellite may process the information and send the information in a communications link in the form of a downlink to a destination terminal in another location across the globe.
In other examples, satellites may relay the information received to multiple destination locations. For example, the information may be a video broadcast received by the satellite in signals for an uplink to the satellite by a transmitter for a user. The satellite may then retransmit this video broadcast in signals in downlinks to the multiple destination locations.
In still other examples, if the destination device is not in the coverage area of a satellite, that satellite may relay the communications to a second satellite via a communications link in the form of a satellite crosslink. The second satellite may then send the communication in a downlink to the destination location.
Users transmitting these types of communications may desire that the communications be protected from interference by others. This interference may be anything which alters, modifies, or disrupts a signal from the transmitter as the signal travels along a channel between the transmitter and the receiver. This interference may be unintentional interference from the environment or intentional interference from others. This intentional interference may be known as “signal jamming.”
Signal jamming is a process of intentionally transmitting radio signals using the same or substantially the same frequencies as those in the uplink, downlink, or both the uplink and downlink to disrupt communication of information by a sender. For example, an adversary may attempt to jam communications signals from an operator at a military ground station to prevent the operator from communicating with troops in other locations. In some cases, users perform signal processing, such as frequency hopping, to protect satellite communications from signal jamming. Users may also perform signal processing. This signal processing may include, for example, without limitation, frequency hopping to protect satellite communications from unintentional sources of interference, and to prevent signal detection, signal interception, or other undesired results.
When relaying communications via satellite, some current and proposed anti-jam systems perform a large part of this signal processing onboard the satellite in orbit. This signal processing may be, for example, frequency hopping, frequency dehopping, time permutation, and time de-permutation. The signal processing also may include, for example, channel interleaving, scrambling, rotation, interspersal techniques, or other types of processing that may be used to increase the security of the communications.
In particular, frequency hopping and frequency dehopping may be used to reduce or avoid interference with communications. In other words, the frequency on which information is carried may be changed over time.
Frequency hopping involves employing a carrier frequency that changes over time. Frequency dehopping involves reversing the process of frequency hopping to identify a carrier frequency that does not change over time in order to enable extraction of the information from the carrier wave.
Signal processing can be a calculation intensive and complex process. As a result, additional equipment may be needed onboard the satellite to perform this signal processing. Consequently, currently used signal processing systems intended for use onboard satellites may increase the size, weight, and cost of the satellite.
Additionally, upgrading or changing signal processing systems may be more difficult than desired. For example, if more sophisticated equipment is needed to perform onboard signal processing on a satellite, a satellite may be modified or replaced. The process of modifying or replacing a satellite may be more time intensive and costly than desired.
In other cases, the increased size, weight, and complexity of a modified satellite may result in undesired or inefficient performance of the satellite. Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as other possible issues.